Laser-diode pumping is perhaps the most active area of current laser development. This technique, which uses semiconductor lasers to replace incoherent optical pump sources such as flash lamps or arc lamps, was responsible for a great deal of the renaissance of activity in solid state laser development that began in the mid-1980s. As diodes became less expensive, more powerful and longer lived, the development of this type of pumping has rapidly increased. Diode pumping allows a more compact laser system with reduced cooling and power consumption. In addition, the development of high-power laser diodes in the red and the infrared wavelength regions have allowed new laser materials to be directly diode pumped.
Many applications have benefited from the development of diode-pumped solid-state (DPSS) lasers. Commercial applications such as cutting and drilling have taken advantage of the longer lifetimes of diode-pumped lasers relative to lamp-pumped devices. Many military and aviation applications require “fieldable” devices, and the long life, compact package, and improved efficiency are strong factors that recommend these types of devices. Excellent examples of such structures are Ring Laser Gyroscopes (RLGs) and turbulence LIDARs, which must operate in extreme environments and are commonly used by space and aerospace companies.
DPSS lasers often have requirements that are conflicting such as resonator length requirements and compact packaging. To be “fieldable,” lasers require good reproducible operation in varying environments. The varying environments can include high vibration, wide thermal swings, shock, and large variations in the input/output requirements. A highly stable mechanical structure that is relatively immune to the local environment such as vibration and thermal enhances reproducibility. Designed in self-compensation for resonator errors such as tilt or translations are highly desirable. Additionally, pumping the laser gain media longitudinally from both sides may be necessary to produce the desired gain from the gain media for a given application. For many laser systems, efficiency is critical and generally, the more efficiently the system performs, the more latitude the system exhibits in performance such as pulse rate and output energy. Low-emission cross-section laser materials amplify these requirements.
Lasers will have a finite bandwidth and a number of modes N within that bandwidth (both axial and transverse); the bandwidth and modes being a function of the resonator length and gain media. The larger a stable laser mode the greater the volume of (energy extraction that occurs within the laser gain medium (for a well mode-matched system). The each of the distinct transverse modes are denoted as TEMmn, where m and n are integers designating which mode. TEM00 designates the lowest order mode of a laser, with a Gaussian energy distribution across the beam.
Generally, it is very difficult to achieve a large stable laser mode size in a resonator that is well within the stable cavity criteria (i.e. the resonator stability coefficient g1g2 is between 0 and 1). One way that the mode can be made larger is by increasing the optical path length. Long optical path lengths, however, are contrary to compact laser systems, but folding the optical resonator with mirrors as internal optical cavity elements can accommodate this.
To create a useable laser mode within a resonator cavity, an optical assembly must rigidly control the location and tilt angles of the most sensitive optical elements, in particular intra-cavity mirrors. Failure to align such optics precisely will diminish the laser energy extraction by creating additional loss, preventing efficient output from the laser. Vibration and thermal swings add variability normally removed in the laboratory environment.
One approach to lengthening an optical cavity was to fold the cavity in a monolithic block structure. Monolithic blocks of low expansion material such as Zerodur™ made by Schott Glas, fused silica, BK-7™ or other glass-ceramic and glass materials have been found suitable to form the optical cavity. The cavity is formed by boring holes in the material to create a path that is optically continuous after the addition of optical mirrors thus producing a folded cavity. Because boring requires a cutter path from the outside of the block inward, designs have generally been limited to single and double folds.
DPSS lasers are often used in Q-switched designs for applications requiring a momentary emission at a high level rather than sustained stimulated emission with the same average power. Optical cavity length is also a key variable in Q-switched laser design. A Q-switched laser modulates the Q of the resonator by maintaining the Q of the cavity at a low value until the desired energy is stored in the laser host media. The Q of the cavity is then rapidly switched to a high value and the energy stored is then largely extracted. The shorter the cavity, the shorter the emitted Q-switch pulse. This scales roughly as the square root of the optical path length. Shorter optical pulses produce greater stress on the optical elements such as mirror coatings. For this reason, as well, longer optical lengths are desirable.
What is needed in the art, then, is a “fieldable” or rugged optical bench for a DPSS laser, having good stability for the optical components with a suitably long optical cavity.